FPS(First Person Shooters) - Call of Duty 4 is great on any platform, Bioshock has a great story and some rpg elements, and ditto with Fallout 3
Racing - Burnout Paradise is great, and they keep putting out even better free updates for it. If you're really into racing sims instead of arcade style, then Gran Turismo is a great franchise.
MK vs. DCU is pretty fun from experience. If you're used to MK gore, you won't find it, but its still a pretty good fighting game. I would recommend Soul Caliber 4 though. As for Street Fighter, I can't say from personal experience, but all my friends that play love it. Just mute it during the opening video...
Metal Gear was amazing. Don't expect to play it in 30 minute intervals though. Some of the cutscenes were upwards of an hour in length. But all in all, the story is amazing, graphics breathtaking, and probably couldn't have tied up the whole metal gear saga much tidier.
